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(USA Today) --Southwest and its subsidiaryAirTran have rolled72-hour domestic fare salesin which ticket prices are set by the length of a flight. With the sale, fares for the airlines' shortest flights will cost less than $100 round-trip, even with taxes and fees included.

The Southwest/AirTran sale comes despite the trend that's seen fares on the rise over the past year. Southwest's sale covers flights during the late summer and fall, a period when demand for leisure travel typically drops as kids head back to school and vacations tail off following the Labor Day holiday.

As for fine print behind the Southwest/AirTran sale, here are the details:

The cheapest fares --$49 each way - are for flights of 500 miles or less. Flights of between 501 and 1,30 miles cost $99 each way while flights of more than 1,301 miles cost $139 each way. The sale covers routes on bothSouthwestandAirTran, though -- despite the merger -- each carrier's flights are not yet available for purchase on the other's website.

The sale is good for travel on flights starting Aug. 13 and running through Nov. 14. The sale fares are not available on Fridays and Sundays. The following dates are blacked out from the fare sale: Aug. 18, Aug. 30, Sept. 3, and Sept. 4. The mileage-based sale fares apply to nonstop flights, and appear to rise by $20 to $40 round-trip on connecting itineraries.

Regardless, bargain hunters must act quickly to secure the deals. The sale ends at 11:59 p.m. PT on Thursday. Additionally, as is typical in airline promotions, only a set number of sale fares are made available on each flight. Once they sell out, fares will begin to rise. A check of Southwest's website showed fairly broad availability of sale fares as of 10:35 a.m. ET.

Sample round-trip fares:

500 miles or less

Baltimore-Buffalo, $97.60
Chicago-Nashville, $96.10
Charleston (S.C.)-Nashville, $96.10
Houston-Dallas, $96.10
Las Vegas-San Diego, $97.60

501-1,300 miles
Atlanta-Chicago, $197.60
Jackson (Miss.)-Baltimore, $197.60
Oakland-Portland (Ore.), $197.60
Los Angeles-Denver, $197.60
Washington National-Austin, $197.60

1,301 miles or more

Newark-Phoenix, $277.60
Los Angeles-St. Louis, $277.60
Seattle-Atlanta, $277.60
Albuquerque-Orlando, $284.70